Generally, a spark plug for use in an internal combustion engine, such as an automotive engine, is configured to ignite an air-fuel mixture supplied into a combustion chamber of the internal combustion engine through generation of spark discharges across a spark discharge gap between a center electrode and a ground electrode.
In recent years, in order to cope with exhaust gas regulations and to improve fuel economy, lean-burn engines, direct-injection engines, low-emission engines, and like internal combustion engines have been actively developed. For ignition of an air-fuel mixture, these internal combustion engines require a spark plug higher in ignition performance than conventional spark plugs.
A known spark plug having enhanced ignition performance has a ground electrode on which a protrusion is formed.
Examples of such a spark plug include a spark plug in which a noble metal tip of an iridium alloy, a platinum alloy, or the like, which exhibits excellent resistance to spark-induced erosion and to oxidation-induced erosion, is welded to an electrode base metal, such as a nickel alloy, of a ground electrode, thereby forming a protrusion, and a spark plug in which, in place of welding of a noble metal tip, the electrode base metal of the ground electrode is machined to form a protrusion (refer to, for example, Patent Document 1).